Ever since the advent of fiber reinforced polymer materials in the field of Aerospace, Metal-FRP stacks started to gain importance due to their superior fatigue performance, phenomenal low weight and good specific strength. However the machining, specially drilling these multi stack materials has always proved to be a challenge for the field of manufacturing and assembly. Drilling holes in only metal with a drill (metal drill), the material removal is through a process of clear shearing since the tool is much harder and sharper than the base material. The tools hence wears at a much slower and gradual rate, also the malleable properties of the machined metal compensate to the reduced cutting capability of the worn out drill. These properties of the machined metal act like a ‘FAIL SAFE’ mechanism during the machining process assuring a trouble free fail safe environment during the drilling process. However, drilling FRP composites is altogether a different story. The very properties that make the FRP composite superior make it a challenge for machining.
This article describes the physical background and the experience in the drilling of carbon fiber and aluminum-carbon fiber stacks. Low temperatures and intelligent chip removal technologies are the most important requirements for dry drilling or to avoid the MQL (minimum quantity lubrication). The drilling in one shot and in IT8 quality is mandatory. In case of machining metal, like aluminum or titanium, a lot of heat is generated by the tools and the cutting process. Machining of composites, the material and the tool should remain as cold as possible even by drilling without external or internal coolant. A new drill design is now developed, qualified and patented by MAPAL that allows the dry drilling of metals at very low temperatures also. We are now able to drill, all batches (composite / aluminum) without MMS. The high drilling feed and due to that, the shorter contact length between the tool and the material stack also gives us approximately twice the tool life. The new tool has already been qualified for drilling stacks in the wing box assembly, without MMS and a CPK value of 2.4.
This technical paper deals with design and manufacture of axial and orbital cutters for drilling large diameters holes in Carbon Fiber Reinforced Plastics (CFRP) / Titanium (TA6V) thick stack by means of an Automated Drilling Unit fixed on a drilling template. Creating tools that drill such stacked holes in a single operation is particularly difficult. The common strategies for tool designs designated to cutting composites stacked with metals as titanium include uncoated carbide, tools with a diamond coating applied by chemical vapor deposition (CVD). It is also question of what kind of drilling process should we use to achieve larger holes in minimum time. Therefore, axial drilling process with pilot, drill and ream steps find a competitor drilling process named orbital drilling which can achieve both operations in one step allowing then, burrless, free delamination, small and easy removable chips all with one tool being able to achieve different hole sizes. The present study compares the quality of drilled holes in term of both materials hole diameters, exit burr and cycle time which become a decision gate for the process choice.